Currently available optical films include (1) low-birefringent optical films having nearly no orientation and having a retardation of 100 nm or less which are useful as a protective film of a polarizing sheet, etc., such as a cellulose triacetate film used as a protective film of a polarizing sheet; and (2) birefringent optical films having a retardation of 1/4.lambda. (130 to 150 nm; .lambda. is a wave length of incident light) which are useful for anti-glare materials, etc., such as a uniaxially stretched cellulose acetate film (e.g., a cellulose diacetate film). The optical film having a retardation of 1/4.lambda. is combined with a linear polarizing sheet with the optical axis thereof being inclined at 45.degree. with respect to the direction of linear polarized light to provide a circular polarizing sheet. Because of glare protection function to cut reflected light, it is used in various anti-glare materials, such as a VDT (visual display terminal) filter. The term "retardation" (R value) as used herein is a product of a birefringence of the film or sheet (}n) and a thickness of the film or sheet (d), i.e., R=}n.times.d.
On the other hand, attempts have been made to improve image quality of TN (twisted nematic) mode liquid crystal displays, in which liquid crystal molecules have a twisted angle of 90.degree. and a pair of polarizing sheets are provided on and under a liquid crystal cell with the absorption axes thereof being crossing or in parallel, which are used in watches or electric calculators by application of a birefringent optical film as disclosed in JP-A-61-186987 and JP-A-60-26322 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
With the recent increasing demands for an increase of display capacity and enlargement of a display screen, a liquid crystal display whose liquid-crystal molecules have a twisted angle of more than 90.degree. (i.e., about 180.degree. to 270.degree.), which is called an STN (super twisted nematic) mode liquid crystal display, has been developed. However, the STN mode liquid crystal display suffers from coloring due to birefringence of liquid crystal molecules and therefore fails to achieve a black-and-white display (hereinafter referred to as a B/W display) that is feasible in conventional TN mode liquid crystal displays. For example, some of the STN liquid crystal displays make a deep blue display on a yellowish green background. Displays assuming such a hue, they often encounter with limitations on making a color display, such as a multi-color display and a full-color display.
In order to solve this problem of the STN mode liquid crystal displays, it has been proposed to add, as an optical compensator, another liquid crystal cell for color removal to a combination of an STN mode liquid crystal cell and polarizing sheets to thereby achieve a B/W display as taught, e.g., in Nikkei Micro Device, Oct. issue, p. 84 (1987). This technique, however, has disadvantages of high cost, an increased weight, and an increased thickness. Therefore, it has been studied to substitute the additional liquid crystal cell with a birefringent optical film.
For the purpose of improving image quality of liquid crystal displays, it has also been studied to apply uniaxially or biaxially stretched birefringent optical films having various retardation values to various liquid crystal displays.
However, the state-of-the-art optical films cannot be used for the new applications including liquid crystal displays for reasons that (1) the retardation values thereof do not optically meet the purpose, (2) the optical axis thereof cannot be arbitrarily controlled, and (3) they suffer from considerable optical color unevenness, typically exemplified by a stripe pattern (hereinafter referred to as streaks), sometimes rather impairing image quality.